Power transmissions in motor vehicles have taken two distinct modes of operation. Automatic shifting transmissions generally include planetary gearsets and are controlled in ratio by hydraulic devices, which in turn are controlled by electro-hydraulic mechanisms. Countershaft transmissions, which include a plurality of intermeshing spur gears or helical gears, utilize mechanical or synchronizer clutches to control the torque and speed path through the transmission. The synchronizers are controlled by mechanical devices, which provide for lateral shifting of the synchronizers to cause engagement between an associated gear member and an associated shaft.
The shifting of the control synchronizers is accomplished by a plurality of shift forks, which in turn are controlled by mechanical shift mechanisms. With these types of shift mechanisms, it is desirable to have some centering apparatus, which will upon release of the shift mechanism in a neutral position cause the shift mechanism to return to a fixed location point.
More recently, synchronizer or mechanical clutches have been suggested for use with planetary-type transmissions. When used with planetary-type transmissions, synchronizers still require a mechanical shift mechanism, which will provide for the appropriate shifting or engagement of the synchronizers and also mechanisms to return the synchronizers to a neutral position when desired by the operator.
In the mechanical clutches or synchronizers that are employed with planetary transmissions, it is quite often necessary to provide for the simultaneous engagement of two synchronizers such that the proper torque path is established within the planetary gearsets.